1. Technical Field
The present invention relates to a resonator element such as a flexural resonator element that vibrates, for example, in a flexural vibration mode, a resonator or an oscillator that uses the resonator element, and an electronic device having the oscillator.
2. Related Art
In the related art, a tuning-fork type flexural resonator element in which a pair of resonating arms is extended from a base portion made from a piezoelectric material such as a quartz crystal so as to vibrate in the direction towards or away from each other in the horizontal direction is widely used as a resonator element that vibrates in the flexural vibration mode. When the resonating arms of the tuning-fork type flexural resonator element are excited, if there is a loss of the vibration energy thereof, it decreases the performance of the resonator element by, for example, an increase of the CI (Crystal Impedance) value or a decrease of the Q value. Thus, various attempts have been made to prevent or suppress such a loss of vibration energy.
For example, a tuning-fork type quartz crystal resonator element in which a cutout portion or a notch (notched groove) with a predetermined depth is formed on both sides of the base portion from which the resonating arms extend is known (for example, see JP-A-2002-261575 and JP-A-2004-260718). In this tuning-fork type quartz crystal resonator element, when the vibration of the resonating arms includes a vertical component, the loss of vibration from the base portion is suppressed by the notch. Thus, a confinement effect of the vibration energy increases, and the Q value is controlled and the fluctuation of the Q value between the resonator elements is prevented.
Moreover, in a resonator element, the decrease of the Q value occurs not only due to the mechanical loss of vibration energy described above, but also occurs due to thermal conduction caused by a temperature difference between a contracted portion where compressive stress of the resonating arms performing flexural vibration acts and an expanded portion where tensile stress acts. The decrease of the Q value caused by thermal conduction is referred to as a thermoelastic loss effect.
A tuning-fork type resonator element in which a groove or a hole is formed on the central line of a resonating arm (vibrating beam) having a rectangular section in order to prevent or suppress the decrease of the Q value due to the thermoelastic loss effect is disclosed in JP-UM-A-2-32229, for example.
In recent years, various electronic devices having the resonator element have been miniaturized. Examples of such electronic devices include miniaturized information devices such as HDDs (Hard Disk Drives), mobile computers, or IC cards, mobile communication devices such as portable phones, car phones, or paging systems, and vibration gyro sensors. In line with this, there is a higher level of demand for miniaturization of resonator elements mounted in these electronic devices.
Among the demand for miniaturization of such a resonator element, shortening of the length of a resonating arm is particularly promising. In line with this, there is a demand to decrease the cross-sectional area of the resonating arm. Therefore, it has been understood that it is difficult to decrease the frequency of the resonator element and the vibration characteristics of the resonator element are likely to become unstable due to the occurrence of a high-order vibration mode. As a resonator element capable of suppressing the occurrence of such a high-order vibration mode, decreasing the frequency, and stabilizing the vibration characteristics, a resonator element in which a weight portion having a larger width than a general portion (arm portion) of a resonating arm is formed at the tip end of the resonating arm is disclosed in JP-UM-A-2005-5896, for example.
However, the present inventor has found that when the resonator element has a structure in which it has a resonating arm which includes both an elongated groove and a weight portion, if the occupancy of the length of the weight portion with respect to the entire length in the longitudinal direction of the resonating arm is too small or too large, it is not possible to obtain a desirable effect of decreasing the frequency with the weight portion and a desirable effect of suppressing thermoelastic loss with the elongated groove. Moreover, the present inventor has also found that when the occupancy of the length of the weight portion with respect to the entire length in the longitudinal direction of the resonating arm is within a certain range, it is possible to obtain the desired Q value by effect of decreasing the frequency with the weight portion and the desirable effect of suppressing thermoelastic loss with the elongated groove.